The disclosed invention is directed generally to hybrid multilayer circuit structures, and is directed more particularly to hybrid multilayer circuit structures having integral electromagnetic interference (EMI) shielding dielectric layers formed therein.
Hybrid multilayer circuit structures, also known as hybrid microcircuits, implement the interconnection and packaging of discrete circuit devices, and generally include a unitized multilayer circuit structure formed from a plurality of integrally fused insulating layers (e.g., ceramic layers) having conductor traces disposed therebetween. The discrete circuit devices (e.g., integrated circuits) are commonly mounted on the top insulating layer so as not to be covered by another insulating layer or on an insulating layer having die cutouts formed thereon to provide cavities for the discrete devices. Passive components such as capacitors and resistors can be formed on the same layer that supports the discrete devices, for example, by thick film processes, or they can be formed between the insulating layers, for example, also by thick film processes. Electrical interconnection of the conductors and components on the different layers is achieved with vias or holes appropriately located and formed in the insulating layers and filled with conductive via fill material, whereby the conductive material is in contact with predetermined conductive traces between the layers that extend over or under the vias.
A consideration with hybrid multilayer circuit structures is shielding and controlling electric fields which are generated internally to the hybrid multilayer circuit structure (for example by RF microstrip or stripline conductors), as well as for externally generated electric fields.
Known techniques for controlling electric fields in hybrid multilayer circuit structures include circuit conductor separation, conductive shielding and/or packaging external to the multilayer circuit structure, and internal conductive ground planes. External shielding adds significant cost in typical applications. Moreover, the required isolation is not always readily achieved with conductive shields wherein ground/shield current flow can induce additional coupling. The problem becomes more difficult with RF power circuits.
A major consideration with conductive shielding is that both the field and induced conductor currents must be considered in controlling internal and external interference and feedback. Any non-orthogonal interaction between a field and a conductor will result in an induced current in the conductor. The induced current will vary at the same frequency as the field, and at RF frequencies the resulting signal is not easily localized and can be easily coupled into circuitry that is sufficiently near the conductor. Where the conductor is a ground, power, or shield plane, the induced signal can be coupled through parasitic elements into virtually any part of the circuitry. This is typically controlled by a combination of providing short, low impedance return paths, separate localized shielding, "point grounds", and modified circuit layouts. The major difficulty is that RF ground currents are not easily predicted or measured, which means that the particular means for controlling induced currents must be determined empirically.